Researchers are interested in lucid dreaming because it can help probe what happens when we switch between conscious states, going from little to full awareness.

In 2010, Ursula Voss at the J.W. Goethe University in Frankfurt, Germany, and her colleagues trained volunteers to move their eyes in a specific pattern during a lucid dream. By scanning their brains while they slept, Voss was able to show that lucid dreams coincided with elevated gamma brainwaves. This kind of brainwave occurs when groups of neurons synchronise their activity, firing together about 40 times a second. The gamma waves occurred mainly in areas situated towards the front of the brain, called the frontal and temporal lobes.

Perchance to dream

The team wanted to see whether gamma brainwaves caused the lucid dreams, or whether both were side effects of some other change. So Voss and her colleagues began another study in which they stimulated the brain of 27 sleeping volunteers, using a non-invasive technique called transcranial alternating current.

Each volunteer came into the lab on many different nights. As they slept, their neural activity was monitored with EEG to identify when they entered rapid eye movement (REM) sleep. On each night, the subject then received electrical stimulation at a different frequency, ranging between 2 and 100 hertz for 2 minutes, or a sham treatment that had no effect on the brain. The participants were then woken up and asked to rate their dream consciousness on a standard scale.

Previous work has shown that during lucid dreaming, three of eight factors on this scale are substantially increased, including awareness of dreaming, control over plot and third-person perspective.

Sure enough, these three factors all increased after volunteers received stimulation at a frequency of 40 Hz, which caused an increase in gamma brainwave activity. Much higher and lower frequencies of stimulation – outside of the gamma range – had no effect on lucid dreaming.

Split consciousness

Researchers believe that our experience of the world is split into two levels of consciousness. Primary consciousness relates to simple emotions and sensory perceptions – a level which could apply to animals as well as humans. Secondary consciousness involves being aware that we are aware – something many would argue is unique to humans.

The gamma wave oscillations may help different areas of the brain synchronise their activity and thereby talk to one another, "binding" thoughts and feelings to create a cohesive experience. Importantly, the frontal and temporal areas – which are involved in decision making and memory and are most highly evolved in humans – aren't normally synchronised in REM sleep, but are in waking consciousness. Ramping up the gamma waves may have created a hybrid state with greater synchronisation and awareness in this area, creating some of the characteristics of secondary consciousness, while the rest of the brain sleeps.

The team suggests that brain stimulation might help people with post-traumatic stress disorder who have recurring nightmares. Perhaps by triggering lucid dreaming, people with PTSD can take control of their dreams and make them less frightening. "That's what we are looking at now," says Voss, although the results are not yet available.

It is a promising suggestion, says Michael Schredl who works in the sleep laboratory at Heidelberg University, Germany. He says it will be difficult to expand the applications to help treat mental disorders, but "the idea of studying patients with nightmares or PTSD would be very interesting".

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